Question: We are experiencing patterns and streaking coming out of our EN (electroless nickel) bath. What can we do to rectify this?
Answer: Oftentimes patterns and streaks are due most commonly to improper or inadequate surface preparation or an under-optimized pretreatment cycle. Failing in one area among the myriad of parameters involved in effective pretreatment can doom a part to a panoply of possible defects. For instance, failure can manifest due to incorrect or inadequate temperatures, high or low immersion times, weak or excessive agitation, high or low concentrations, old or spent solutions, excessive or quickened transfer times, and other integral and essential aspects of suitable cleaning protocol. Sometimes these malfunctions can be due to human error wherein the operators (rackers and unrackers mainly) fail to simply wear clean cotton or latex gloves. Fingerprints, especially on low-carbon steel substrates, for instance, are notoriously difficult to clean and have the potential, due to their acidity and moisture levels, to etch into the metal base material, leaving a pattern that produces a very defined and unavoidable defect. Cleaners that are too hot or too cold can often leave a dry-down pattern on parts, most easily and readily on larger items versus smaller ones, but the phenomenon can appear without consideration to size if the offending pretreatment tank is poorly maintained or the offending contaminant is prominent and tenacious enough to manifest in a particularly insidious way. Beyond the alkaline solutions discussed above, the acid pickling or activation solutions can be a problem too. Acids that are too aggressive can etch and smut up the surface if they lack an inhibiting agent and this excessive pickling often draws the metallic alloying constituents (often carbon) and even the inevitable impurities of the substrate to the surface, which can not only contribute to patterning, but also potential catastrophic adhesion failure.
It is important to observe and inspect the parts at each step in the pretreatment cycle to ensure parts appear as they ought to appear. Oil rundowns, smutting out of the acid activation tank, paraffin residuals, and many other possibilities abound in the pretreatment sphere as mentioned previously and may be more less evident to the untrained eye, which is why consistent monitoring by an experienced operator is key to ensuring quality. Examination of the substrate material and a thorough understanding of the manufacturing process for each part including type of process, forging, stamping, casting, machining, etc., the types of lubricants, oils, greases, and so on that were used, and the substrate type, brass, bronze, aluminum, or steel and the various iterations within these larger categories, is key to optimizing the pretreatment cycle to certify, validate, and confirm that cleaning is ideal, but not excessive.
Water breaks are a good indication, but not the only indication, of inadequate pretreatment. Parts can also be wiped with a clean, white rag to ensure no residual greases, oils, soils, or smuts remain on the parts prior to their entrance into the plating tank. These inspections should be completed right before the parts enter the EN plating electrolyte, which is usually after the acid activation rinse tank.
Sodium metasilicates are used to add to the alkalinity and soil dispersion of the soak and electrocleaner tanks. They also buffer the pH to mitigate against acid drag-in, which is most commonly an issue in double-cleaning cycles. Additionally, silicates help to prevent flash rusting and prevent redeposition of soils once removed. These ingredients are great additions to any cleaner package, however, if not properly controlled, can produce, often when coupled with hard water environments (high levels of calcium and magnesium), problems associated with streaking due to the formation of a gelatinous siliceous film when they encounter acidic material.
Failure to keep the post-plating rinses clean and properly heated can also cause superficial patterns and streaks that can be alleviated, oftentimes, with the introduction of ammonium hydroxide either via immersion or using an ammonia-soaked rag to wipe the offending areas.
Flash Rusting
Flash rusting also has the potential to cause plating issues beyond streaking, like adhesion failure. Flash rusting with steel occurs usually when the caustic in either cleaner tank (soak or electro) is low. A standard phenolphthalein titration is usually adequate for tanks that have been made up fairly recently, but the level of sodium or potassium carbonates that develop in most electrocleaners, but most commonly with steel ones, tends to grow over time as they are continually and consistently exposed to oxygen in the atmosphere and from the evolution of oxygen at the anodes from the electrolytic reaction. This causal nexus then gives a falsely high alkalinity figure that may produce the impression that the caustic content is adequate, when in reality it is severely underdosed and may be the cause of the flash rusting and also potentially inadequate electrocleaning activity due to the low conductivity of the carbonate ion in relation to the higher conductivity of the hydroxyl ion.
Double-cleaning cycles can also introduce chlorides from a hydrochloric acid pickle into the electrocleaners which can act as a powerful catalyst for etching parts as the electrocleaning mechanism becomes overly aggressive due to the high conductivity of the chloride ion. Excessive conductivity can be as big of a problem as inadequate conductivity, if not even more so.
Beyond pretreatment, it is important to keep the EN bath as clean as possible. Poor pretreatment is not only bad because it fails to condition the part for good adhesion and cosmetically-appealing deposits, but it also keeps those oils, soils, and greases out of the most expensive tank on the line, the EN tank. Most EN baths should be achieving approximately 6-10 metal turnovers (MTO’s) or more, but failure to halt the introduction of both organic and metallic elements into the bath will most assuredly decrease that average MTO, therefore impacting the overall longevity, efficiency, effectiveness, and economy of your entire EN system.
Nitric Acid Stripping
Nitric acid stripping is the most popular method for stripping and passivating an EN tank, but the potential for nitrate and nitrite contamination leads many to opt for the sulfuric acid/peroxide strippers instead. Failure to properly cleanse the bath of nitric residuals results in dark, dull, and yes, you guessed it, streaked and patterned deposits. Be sure to rinse your plating tank after a nitric strip thoroughly at the same temperature and, preferably, the same surface tension (to penetrate all those little nooks and crannies) so as to ensure the plastic has not swelled or contracted beyond what it would at the stripping temperature. As you can imagine, small amounts of nitric may be trapped in certain areas of the plastic that will then release once the solution temperature of the EN is brought into the 180-190 degree plating temperature, which would then cause an elevated nitrate level in the bath, which will invariably stifle the plating rate and cause the patterns and streaks mentioned before.
If the pattern begins to look like stripes of a tiger this is due most often to high levels of stabilizing agents, particularly of the metallic variety. If the defect exhibits as white stripes and streaks, not dark ones, oftentimes this is due to LOW metallic stabilizers. As you can observe patterns and streaks manifest in different ways if your metal stabilizers are out of balance. Again, if residual nitric acid is left in the tank, or if copper from substrates and racking components dissolve into the EN solution, both of these components can and will consume sulfur stabilizers. Sulfur stabilizers are very common stabilizing components especially in the low, low-mid, and mid phos (phosphorous) bath formulations. The consumption of sulfur stabilizers would cause a major imbalance in the metallic stabilizers and produce the patterning effects just described.
If a more thorough test via a polarograph, ICP, AA, or some other more sophisticated instrumentation is used to determine stabilizer content and it is shown to be out of balance, it is important to know that most baths can have their imbalance corrected. When organic stabilizers have been consumed or there is simply an excess of metallic stabilizers due to contamination or imprecise add backs the operator can apply current to the EN bath to plate-out the metallics as you would with tramp metals in a standard electrolytic plating bath like a Watts nickel to remove copper, lead, or other metal contaminants. If you discover that the sulfur content is high, and this is the cause of the patterns, you can hang and immerse copper slugs, balls, or panels into the tank to consume the sulfur.
Diluting the Bath
Of course, as with all things, metallic excesses can be alleviated simply by diluting the bath. Adjustments of the other components, nickel metal, hypo (sodium hypophosphite), etc. will need to be made separately, of course, using specialty tools with the pure components. Oftentimes EN baths are two-component systems with an A and a B addition to be made based on consumption rates, but the “B” or secondary component will contain not only hypophosphite, but also all of the other crucial ingredients, including stabilizers and therefore, after you have finished equalizing and balancing your bath, you want to be sure to procure pure hypophosphite from your chemical vendor so as not to put yourself in the same situation you just saved your bath from previously.
It is important to know and to consistently achieve proper loading for your EN bath. This is usually, for most formulations, between 0.25-0.75 ft2/gal. There are many baths that can range all the way down to 0.05 ft2/gal and even possibly 0.01 ft2/gal and beyond, and all the way up to 1 ft2/gal and beyond, but oftentimes they need to be formulated differently with a different recipe or concoction of stabilizing agents to achieve these extremes. For the average EN bath this would mean that if you have a standard 100-gallon solution you should be aiming to plate with 50 square feet of parts to achieve the optimal deposition quality and other benefits like plating rate and brightness. Failure to reach the loading rate may result in edge defects that is a product of an over-stabilized bath. These edge-pullback defects can create a patterned and streaky surface as well. A good rule of thumb to remember is that an EN bath is like a husky dog in the Iditarod. They feel happy and stable when they are being worked at just the right level. To remember this a little easier, try to think of the children’s story, Goldilocks and the Three Bears. EN baths should be run just right, not too hard and not too little.
Sometimes the pattern is less of a streak and more of a gas pattern that can mimic a streak, but is most certainly its own unique pattern. As the EN bath is plating it evolves a voluminous amount of hydrogen gas. If the solution’s surface tension is considerably high or simply not low enough, the interfacial tension at the surface of the plating object will be high and therefore the ease with which the hydrogen bubbles that generate is removed from the object is lessened. It is therefore imperative to ensure the bath is not overly viscous and therefore slick enough to mitigate the potential effects of gas patterning and pitting. A dispersant can also help mitigate other potential contaminants that can interfere with the clarity of the deposit itself and often have dual properties of dispersion and wetting, which lowers surface tension.
Solution Transfer
Another point to consider regarding gas patterning is the importance of solution transfer and replenishment of fresh solution to the areas undergoing deposition. Those areas with poor agitation will produce a different type or quality of plating than an area with proper or good agitation. It is important that all areas of the part are replenished regularly with fresh solution with the proper levels of hypophosphite and nickel metal along with the other proprietary constituents. Agitation can come in the form of eductors, air spargers, cathode rod movement, or simply a good, old-fashioned shake at periodic intervals on those manual lines that can pursue this option.
Lastly, filtration is not to be ignored. Poor filtration may manifest more demonstrably in the form of nodules from unfiltered particulates and thus the filtration system may need to be inspected to ensure pore size, media compatibility, solids-holding capacity, turnover rate, and other important facets of proper filtration should be investigated and optimized.
As is clear, there are many reasons why your parts might be exhibiting a patterned or streaky appearance. The patterns and streaks may be generating due to an inadequate pretreatment cycle, poor rinsing, poor water quality, excessive or inadequate pretreatment ingredients, chemical interactions, organic or metallic contamination in the pretreatment cycle or beyond, low or high metallic stabilizers in the EN bath, low or high loading rates in the EN bath, high or low surface tension, poor filtration or low dispersant concentrations, and a multitude of many other possible issues. As is apparent and palpable, maintenance of each step of the plating cycle is crucial and non-negotiable to achieve a high-quality electroless nickel deposit.
Adam Blakeley, MSF, CEF is Director of Eastern Region Technical Services for MacDermid Enthone Industrial Solutions. Reach him at Adam.Blakeley@macdermidenthone.com and visit macdermidenthone.com
